A laser is a light amplifying apparatus which produces coherent monochromatic light with excellent directivity. Dye lasers are a class of liquid lasers. They have an optical resonator comprising a transparent cell which contains a solution of a laser active dye. They also comprise a pumping energy source optically coupled to the cell. During laser operation, the dye solution can be circulated through a circulation system which includes the cell.
Typical energy pumping sources emit high energy electrons or light. Discharge tubes, flash lamps, liquid lasers, gas lasers, and solid lasers can be used as energy pumping units. As a result of their action, the dye molecules in the dye lasers are excited to higher energy states causing radiant energy transformation. The light produced, which travels along the axis of the resonator, is confined within the resonator for a sufficient period of time to strongly interact with the excited dye molecules. When the number of excited dye molecules exceeds the number of molecules in the ground state, induction emission occurs, and the light is amplified within the resonator to emit laser light.
One of the major advantages of the dye laser over solid and gas lasers is its tunability with respect to output wavelengths; that is, although the laser active dye has a certain range of fluorescent band, its output wavelengths are accurately controlled with the aid of a suitable device, such as a prism or a diffraction grating.
Laser dyes are commonly employed in alcoholic solutions, even though the thermal properties of water are superior to those of any alcohol. Specifically, the variation of the refractive index of water is smaller than that of ethanol. This characteristic is particularly important for the development of high-power lasers and for continuous wave lasers.
Aqueous dye solutions have usually not been used in dye lasers because of low dye solubility and the formation of dye dimers and higher aggregates. It is most common for dye dimers and aggregates to form in the H-configuration; such dimers and aggregates usually show greatly diminished fluorescence quantum yields which are incompatible with effective lasing. Even for those cases where dye aggregation does not decrease fluorescence, there are spectral shifts between dye monomers and dye aggregate to contend with. Apparently for these reasons, Applicant is unaware of any practical applications involving dye dimers or higher aggregates in dye lasers. Thus, a need exists for dye laser aqueous systems which have enhanced dye solubility and which combat dye dimerization and higher aggregation. This invention satisfies those needs.
As part of this invention, it has been shown that the presence of a substituted cyclodextrin increases the water solubility of lasing dyes. The greater solubility apparently assists in increasing the fluorescence level, thereby making the system meet the criteria necessary for effective lasing. This ability of substitution on the cyclodextrin ring to increase the fluorescence level is unknown in the art.
As explained more fully below, this invention comprises the use of substituted cyclodextrins to combat dye aggregation. The synthesis of chemically modified cyclodextrins is extensively discussed in Tetrahedron 37, No. 9, pp. 1417-1474 (1983). As stated therein, cyclodextrins are cyclic oligosaccharides consisting of at least six glucopyranose units which are joined together by .alpha.(1.fwdarw.4) linkages. Although cyclodextrins with up to 12 glucose residues are known, only the first three homologs, i.e., those having 6, 7, or 8 glucose units, have been studied extensively. The oligosaccharide ring forms a torus, i.e., a truncated hollow cone, with the primary hydroxyl groups of the glucose residues lying on the narrow end of the torus. The secondary glucopyranose hydroxyl groups are located on the wider end.
The initial discovery of cyclodextrins is attributed to Villiers, who isolated them as degradation products of starch. In 1904, Schardinger demonstrated that these compounds could be obtained by the action of Bacillus macerans amylase upon starch.